Tubular pipe goods used in the oil field are typically inspected for hidden flaws and other defects in the pipe walls by electromagnetic and radioactivity inspecting techniques. With respect to radioactivity inspection techniques, one such technique involves measuring radioactivity which is back scattered from a pipe wall segment. In another radioactivity technique, radiation is passed diametrically through the pipe from a source to a detector and measures the cummulative wall thickness of wall segments at 180 degrees from one another. In still another radioactivity technique, the detector is located on a probe within the pipe so that radioactivity passes only through one wall segment.
In any of the foregoing inspection techniques, the tubular pipe is moved, longitudinally and rotatively, relative to the inspection device so that the inspection covers a helical section of the pipe. By adjusting the relative speeds, this inspection obtains adequate measurements.
One other factor of importance to inspection of pipe is its "eccentricity", i.e., the relative offset of the inner cylindrical wall surface to the outer cylindrical wall surface. Eccentricity occurs in pipe for a variety of reasons and becomes important when the pipe will be used in an oil or gas well. It is commonly accepted that a figure of 12.5% or less of eccentricity is acceptable. For example, with a 0.5 inch nominal wall thickness, the wall thickness is acceptable if it does not exceed plus or minus 0.0625 inches. Eccentricity is measured by the shift of the inner cylindrical surface relative to the outer cylindrical surface.
When a pipe is longitudinally and rotatively moved relative to an inspection head, tolerances of the equipment dictate that the pipe axis can be offset with respect to the axis of an inspection head. If the equipment tolerances cause an offset of the axes it is possible to have a situation where a concentrically perfect pipe produces a measurment similar to a measurment made for an eccentric pipe where the pipe and equipment axis are centered. In either instance, the measurement made through both sections of a pipe is actually a composite measurement including equipment offset affects and effects due to pipe eccentricity.
The present invention in one sense, involves a measurement through both walls of a pipe but is improved so as to eliminate source of weakness which is inherent in a system which passes diametrically through a pipe, this weakness being the inability to detect true eccentricity.
With respect to the apparatus for a typical radioactivity measuring system which diametrically measures through a pipe there is a radioactivity detector and a radioactive source diametrically located relative to one another on either side of a pipe. Typically, the source and detector are mounted on a vertical plate with a central opening so that as the plate is rotated about its axis, a pipe with its axis central with plate axis is longitudinally moved through the rotating plate. This system produces wall thickness measurements along a general helical path about the pipe. It would probably be ideal to move the pipe so slowly and rotate the radiation devices so fast that all of the pipe wall was inspected but as a practical matter this is not necessary and a helical path of inspection is an acceptable procedure.